In this disclosure, the term “wireless device” is used to represent a communication entity capable of radio communication with a wireless network by sending and receiving radio signals. The wireless device described herein may, without limitation, be any of a mobile telephone, a tablet, a laptop computer and a Machine-to-Machine, M2M, device, also known as Machine Type Communication, MTC, device. Another common generic term in this field is “User Equipment, UE” which may be used herein as a synonym for wireless device.
Further, the term “network node”, is used herein to represent any node of a wireless network that is operative to communicate radio signals with wireless devices. The network node in this disclosure may refer to a base station, radio node, Node B, eNB, base transceiver station, access point, etc., although this disclosure is not limited to these examples either. The network node in this disclosure may also be a Radio Network Controller, RNC, or similar that controls one or more base stations or radio nodes that communicate radio signals with wireless devices. The term “base station” may be used herein as a synonym for network node.
Also, the term “radio node” used herein may represent a wireless device or a network node.
Beamforming is likely to become an important technology in future radio communication systems. Multiple antennas can be used for both transmission and reception, referred to as Multiple-Input-Multiple-Output, MIMO, which enables the use of precoders to accomplish beamforming. Beamforming may improve performance in a wireless network, by increasing the received signal strength, thereby improving the coverage, and by reducing unwanted interference, thereby improving the capacity. Beamforming can be applied both in a signal transmitting node, referred to as a transmitter, and a signal receiving node, referred to as a receiver. In a transmitter, beamforming amounts to configuring the transmitter to transmit the signal in a specific direction and not in other directions. This can be achieved by applying a specific precoder to the transmission. By knowing which precoder has been used in the transmission, the receiver is able to apply the same precoder when receiving the transmission.
Different precoders thus produce different beam directions and there are a number of predefined precoders to choose from when selecting which precoder is best to use in a particular communication, referred to as a precoder codebook. During the communication, the available precoders are evaluated based on signal measurements and if the measurements indicate that there is another precoder which is better than the currently used one, the new precoder is used instead. The above evaluation can be performed by the wireless terminal when the network node transmits certain reference signals on different beams, by measuring received power of a reference signal using different candidate precoders. This process is referred to as beam tracking.
In many wireless communications systems CSI (Channel-State Information) feedback is crucial for obtaining good performance. Typically, reference signals transmitted by the network node are used by the wireless device to estimate the channel state, whereupon the reported CSI feedback typically includes CQI (Channel-Quality Indicator), RI (Rank Indicator), and PMI (Precoding Matrix Indicator) values.
The 3GPP LTE (Long Term Evolution) system supports CSI-reporting schemes that rely on the reference symbols being transmitted periodically; cell-specific reference symbols (CRS) are sent every sub-frame while user-specific CSI-RS can be sent with a larger periodicity.
In LTE, open-loop and closed-loop MIMO are two spatial multiplexing schemes supporting multi-layer data transmissions. For closed-loop MIMO the CSI feedback comprises a PMI indicating a preferred precoder from a precoder codebook. The closed-loop MIMO scheme require accurate channel knowledge since if the wrong precoder is used, the performance of the scheme is bad.
For single-layer transmission the open-loop MIMO scheme uses transmit-diversity while for multi-layer transmission a precoding cycling scheme is used wherein a set of precoders are cycled over the sub-carriers on the scheduled resource. For the open-loop MIMO, the CSI feedback hence does no need to comprise a PMI and it has the benefit that it provides diversity in cases when the channel is not known in detailed or when the CSI is less accurate.
Future access technologies are expected to support a large number of transmit antennas, especially on the network side. In the context of Massive MIMO as an example, the number of antennas is expected to be large, where a single transmission point could have in the order of several hundreds or even thousands of antenna elements. A fairly large number of antennas could potentially be expected also in the terminal at the high carrier frequencies, since the physical size of the antenna elements at those frequencies can be made very small.
The increased number of antenna elements makes it possible to form increasingly directive antenna patterns as compared to what is possible with the older antenna systems of today. The transmitted and/or received signal can thereby be focused much more efficiently towards the wireless device, whilst suppressing the interference from and to other wireless devices. Each such direction is typically referred to as a “beam”, whereas the entire process is referred to as “beam-forming”.
Beamforming may be viewed as precoding wherein a beam corresponds to a precoder. It is also possible to perform precoding within a beam, for example if the beam determines an angular direction from a transmission point the transmission in the angular direction may be a precoded transmission. A precoded transmission within a beam may be described mathematically as the product WP, where W is precoder for the beam and P is precoder within the beam. In fact, the 3GPP R-13 precoder codebook has the property that each of the precoders within the codebook is a product of two (component) precoders.
FIG. 1A illustrates that a wireless device 100 receives and measures reference signals transmitted by a network node 102 in different beams using different corresponding precoders P1, P2, P3, P4, P5 . . . . A predefined sequence of such beams is typically repeated at regular intervals. This way, the wireless device 100 can evaluate the precoders based on the measurements and indicate to the network node 102 which precoder is the best and preferred one, e.g. by sending a CSI feedback comprising the above-mentioned PMI. FIG. 1B illustrates that the wireless device 100 and the network node 102 can then start to communicate data using the preferred precoder Pc as long as the wireless device 100 is within its corresponding beam Bc.
However, it is a problem that the above-described evaluation of precoders typically takes a substantial amount of time, e.g. when the received power is measured for the available precoders one by one, particularly if each individual precoder is used infrequently for transmitting a reference signal. The network node also needs to set up radio resources for the reference signals. It is therefore a problem that the radio conditions may change rapidly such that the measurements will be shortly outdated and misleading, i.e. useless, before all possibilities have been measured and evaluated. The performance can thus be deteriorated by selecting a precoder based on outdated measurements. Another problem is that precious radio resources are occupied by the reference signals which reduces the amount of radio resources that can be used for data transmissions, and additional signaling is also required for configuring the reference signals.